Recently there has been an increased awareness of the many benefits associated with being physically fit. One particular form of exercise used ro achieve and maintain physical fitness has been running or jogging.
Most joggers have a running style in which the heel strikes initially, after which the foot is rolled successively forwardly to the mid-part of the foot, tne metatarsals and the toes which provide tne impetus for the next stride. A minority of joggers simply land substantially on the full lengtn of the foot.
A recent study estimates that 34 million Americans run or jog on a regular basis. The exercise of jogging is accompanied for many runners with nagging muscular and skeletal injuries such as tendonitis, shin splints and joint problems, especially in the knee and foot. The upper and lower leg and intermediate knee joint actually operate as a shock absorber during running. Since the knee joint is one of the most fragile joints of the body, it is very desirable to reduce the forces acting upon the knee joint as much as possible.
The muscular and joint skeletal injuries are augmented by the fact that running causes a force of two to three times the body weight on the runner which must be absorbed by the muscular-skeletal structure of the leg and body. An isolated impact causes minimal harm, but for a jogger who runs several miles every day, the repetitive forces frequently cause muscular and joint discomfort in the knee, foot and leg.
Some runners are so-called "full foot strikers" while others are so-called "heel strikers". The "heel strikers" run so as to cause the heel initially to contact or strike the ground. Thereafter, the foot is rolled forwardly into successive ground contact throughout its entire length, and " push off" for the next stride is provided by the metatarsal and toe areas of the foot. The "full foot strikers" run so that the entire length of each foot strikes the ground at substantially the same time, and "push off" mechanics for the next strike is generally accomplished as in the case of "heel strikers" pointed out above.
In either case, depending upon the muscular skeletal structure of the foot, knee, upper and lower leg, and hip, the upright or vertical axis of the foot is generally not in alignment with the vertical or upright axis of the lower leg. When the foot axis is inclined inwardly of the leg axis, this condition is referred to as "pronation" of the foot. When the foot axis is inclined outwardly of the leg axis, this condition is referred to as "supinaiton" of the foot.
The majority of runners have a foot and leg conformation causing pronation of the foot. Limited pronation (up to an angle of 10.degree.) is believed to be desirable as being a natural motion which absorbs some of the impact forces when a jogger's foot strikes the ground. However, excessive pronation exerts excessive force and torque on the foot and knee to a magnified degree often resulting in injury, muscular discomfort and joint discomfort in the knee, foot and leg.
The minority of runners have a foot and leg conformation causing supination of the foot. Again, from a force and torque standpoint as pointed out above, excessive supination likewise causes the undesirable effects referred to above.
For the past decade designers have attempted to create jogging shoes by striking a balance among the factors of durability, cushioning, reduced weight, flexibility and stability to the heel and foot itself. Many prior art designs of running shoes utilize cushioning material in an attempt to ease the forces applied to the knee, foot and leg during running. A cushioning material commonly used in the midsole and heel areas of a shoe is a plastic foam formed from a mixture of ethylene vinyl and acetate. This plastic foam includes a myriad of tiny, variable sized gas bubbles, each bubble being encapsulated by a wall of the plastic foam. Such a plastic foam absorbs shock forces in running but has exhibited poor durability because of the breakdown in the plastic foam walls during use which greatly decreases the cushioning property of the plastic foam. Furthermore, such plastic foam materials exhibit relatively high degrees of hysteresis during loading resulting in inefficient use of energy and do not provide a sufficient degree of stability to the foot during running.
More recently shoe midsole and heel materials are being made from polyurethane foam. This foam also contains a myriad of tiny air spaces somewhat in bubble form but these air spaces are connected by tortuous pathways somewhat like the interstices of a sponge. This type of structure allows air to migrate from one portion of the midsole to another when the foam is compressed. It is more durable than the bubble plastic foam, but is firmer and has less of a cushioning effect. Additionally, it has the same deficiencies as other elastomeric materials, as pointed out above.
Alternate prior art shock absorbing means for shoes utilize various forms of metal springs, such as coils or cantilevers, or pockets of oil or air in the heel area of the shoe. However, results obtained from any of these methods have proven only partially satisfactory, often resulting in a shoe more difficult and expensive to produce.
Further, none of the aforementioned shock absorbing means provide a structure which will substantially and effectively cradle the heel during jogging; which will provide added stability to the heel and foot during running; which has a useful life greater than the life of the shoe; and which has predictable resilient flexibility to cushion the heel and thereafter reapply a substantial portion of the absorbed force to the heel during a stride in running.
The present invention is directed to overcoming one or more of the problems as set forth above.